The advances in digital imaging and printing technologies have vastly improved desktop publishing, yet have provided counterfeiters with lower cost technologies for illegally counterfeiting security and value documents, like identity documents, banknotes, checks, etc. While there are many technologies that make counterfeiting more difficult, there is a need for technologies that can quickly and accurately detect copies. Preferably, these technologies should integrate with existing processes for handling the value documents. For example, in the case of value documents like checks, there is a need for copy detection technology that integrates within the standard printing and validation processes in place today. Further, as paper checks are increasingly being scanned and processed in the digital realm, anti-counterfeiting technologies need to move into this realm as well.
One promising technology for automated copy detection is digital watermarking. Digital watermarking is a process for modifying physical or electronic media to embed a hidden machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
In the case of value documents, digital watermarking can be applied to printed objects for copy detection. In some applications, the digital watermarking techniques can be generalized to auxiliary data embedding methods that can be used to create designed graphics, features or background patterns on value documents that carry auxiliary data. These more general data embedding methods creates printable image features that carry auxiliary data covertly, yet are not necessarily invisible. They afford the flexibility to create aesthetically pleasing graphics or unobtrusive patterns that carry covert signals used to authenticate the printed object and distinguish copies from originals.
Auxiliary data embedding systems for documents typically have two primary components: an encoder that embeds the auxiliary signal in a host document image, and a decoder that detects and reads the embedded auxiliary signal from a document. The encoder embeds the auxiliary signal by subtly altering an image to be printed on the host signal or generating an image carrying the auxiliary data. The reading component analyzes a suspect image scanned from the document to detect whether an auxiliary signal is present, and if so, extracts information carried in it.
Several particular digital watermarking and related auxiliary data embedding techniques have been developed for print media. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible digital watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 6,614,914 and 6,122,403, which are hereby incorporated by reference.
This disclosure describes methods for using embedded auxiliary signals in documents for copy detection. The auxiliary signal is formed as an array of elements selected from a set of print structures with properties that change differently in response to copy operations. These changes in properties of the print structures that carry the embedded auxiliary signal are automatically detectable. For example, the changes make the embedded auxiliary signal more or less detectable. The extent to which the auxiliary data is detected forms a detection metric used in combination with one or more other metrics to differentiate copies from originals. Examples of sets of properties of the print structures that change differently in response to copy operations include sets of colors (including different types of inks), sets of screens or dot structures that have varying dot gain, sets of structures with different aliasing effects, sets of structures with different frequency domain attributes, etc.
Further features will become apparent with reference to the following detailed description and accompanying drawings.